OLS-SG-230, Wood Fired & Salt Glazed Blue Vase

Vapor Glazing

Salt Glazed pottery incorporates a unique method of glaze application.   Instead of the potter applying a coating of glaze minerals onto the surface of the pot before firing, during the salt glazing process pots are engulfed in a sea of sodium vapors and fire.  When salt is added into the kiln at 2000 degrees, releasing sodium which acts as a flux on the silica in the pots, the vapors melt the surface of the pottery.  The process is called “vapor glazing.”

The technique dates to 15th century Germany. The discovery of the process likely resulted from using salt-soaked driftwood or brine-encrusted staves of barrels as fuel in wood-fired kilns.

Adding Salt to the Kiln

This photo shows Bryan adding salt using a length of angle iron filled with salt that is slid into the firebox and dumped.

In his new larger Noborigama Robert adds salt by wrapping it in newspaper creating “salt burritos,” which are added with the wood into the firebox,  as his preferred method.   The amount of salt used depends on the intent of the potter.  Adding a pound of salt per cubic foot of kiln volume is usual for potters seeking deep texture or what is known as an “orange peel” surface.

Wadding

The sodium and fly ash from the wood, create a glaze on the pots, and the kiln shelves, where the pots sit.  Wadding prevents the pottery, in a wood firing or salt glazing, from fusing to the kiln shelves.  The size, shape, and materials used for wadding, are important aesthetic considerations for the potter.

In addition to preventing pots from fusing to shelves, wadding acts as a color resist and leaves flame flashing patterns.
The unusual texture on the underside of this pot results from the method Robert used to cut it free of the potter's wheel. Utilizing a twisted cord, Robert drags the cord under the freshly thrown pot as the wheel is slowly rotating.  This action separates the pot from the wheel head and produces a vibrant pattern.

After the salt firing a silicone carbide shelf looks like this.

After the Firing

The pattern on the floor results from sodium vapors glazing the bricks.  There is a tendency for the gases to be pulled toward the center of the chamber, as seen in the image above.  This unevenness can be countered with the design of the bag wall and the placement of pots and posts.  The firebox is on the right of this photo and a diminishing amount of glaze appears as it travels from right to left.

Firing the Noborigama 60
Firing the Noborigama 60.

Noborigama 60, 1992-2007

This 60 cubic-foot kiln was constructed on an existing concrete slab.  Since the slab was flat, the inclination of the chambers was accomplished by using cement blocks.  There was an unexpected advantage from this design element, as the stoke holes for the salt chamber were at eye level, which made for easy feeding.

Adding Salt
Adding salt using a piece of angle iron.

Standing Up for Wood Firing

The height of this firebox is at eye level, making stoking easy on the back. It also means the feeder stands below the heat and smoke. “Standing Up for Wood Firing” was an article written by Robert for the International Conference of Wood-Fired Potters.

Raking the coals
Raking the coals

Raking Coals During a Firing

An aspect of this particular kiln was its lack of firebox grates. Coals tended to build up, thus reducing the supply of air. Raking the firebox became a part of the firing cycle in this kiln.

Bryan adding salt.
Bryan adding salt.
Bryan Mattraw, an apprentice at our pottery for three years, is shown here stoking & salting the second chamber. Salt is added by putting it on an oak slat, thus feeding the fire and creating a sodium infusion.
Noborigama 60
Noborigama 60
This kiln was originally built without a cover and remained that way for several years. Putting up this structure gave a protected area for wood storage and a space for a Pit Kiln. This modest-sized wood kiln served Robert's studio well for fifteen years. It fired easily in about twenty hours, one of the drawbacks was its small size, which prohibited firing large pieces. Robert built his Noborigama 250 cubic feet to solve those issues.
The Noborigama 60 covered in snow.
The Noborigama 60 covered in snow.

A Winter Blanket of Snow

The wood kiln sleeps during the winter. This quiet time at the pottery allows Robert to throw pots for the coming summer. This yearly rhythm allowed for firings to occur in the spring, summer, and fall. The first firing is usually in June and the last firing is in September.